Ask me next week and it'll probably be a different five:

1.) THE CONTRACT WITH GOD TRILOGY by Eisner

2.) DOCTOR STRANGE MASTERWORKS vol 1 by Lee & Ditko

3.) THE BWS CONAN ARCHIVES vol 1 by Thomas & BWS

4.) DOOM PATROL ARCHIVES vol 1 by Drake & Premiani

5.) WATCHMEN by Moore & Gibbons